1. Field of the Invention
The present invention relates to an audio tandem relay device for transmitting and exchanging an audio signal in a communication system which employs synchronous transfer mode (STM) and an asynchronous transfer mode (ATM).
2. Description of the Prior Art
FIG. 1 is a block diagram showing the structure of a conventional audio tandem relay device. In this structure, a cell disassembly 1 disassembles an ATM cell inputted from a line into an encoded audio signal; an audio decoder 2 decodes an encoded audio signal into an audio signal; an audio encoder 3 encodes an audio signal inputted from an exchange side into an encoded audio signal; a cell assembly 4 assembles an ATM cell from an inputted encoded audio signal, etc.; a pseudo 64k signal composer 5a generates a pseudo encoded audio signal of 64 kbit/s (a pseudo audio signal) by converting, without decoding, an encoded audio signal of 32 kbit/s or 16 kbit/s, etc., the pseudo audio signal maintaining the same data rate as that before the conversion and being usable by a relay exchange; a pseudo 64k signal decomposer 6a changes a pseudo 64k signal inputted from an exchange side into an encoded audio signal having the original data rate; a first pattern inserter 10 inserts a first pattern signal so that an audio tandem relay device which makes a pair with its own audio tandem relay device in relaying (a paired audio tandem relay device or a paired device) will acknowledge that the current signal transmission is a relay transmission; a second pattern inserter 11 inserts a second pattern signal so that a paired device will acknowledge that its own device is in a relay exchange state after detection of a first pattern signal; a first/second pattern detector 12 detects either a first or second pattern signal inserted by a paired device; a second switch 20 switches its connection from a contact point 2 to a contact point 1 upon detection of either a first or second pattern signal in a signal inputted from an exchange side, to thereby output a signal from a pseudo 64k decomposer 6a in place of that from an audio encoder; and a first switch 21 switches its connection upon detection of either a first or second pattern signal in a signal inputted from an exchange side, to thereby output a signal from the second pattern inserter 11 in place of that from the first pattern inserter 10.
An operation will next be described based on the accompanying drawings. FIG. 1 shows an audio tandem relay device in the initial state, in which first and second switches 20, 21 are connected at contact points 2. When a telephone makes a call, an exchange to which the phone is connected outputs a Pulse Code Modulation (PCM) audio signal of 64 kbit/s into an audio tandem relay device connected thereto. As this signal includes neither a first nor a second pattern signal, a first/second pattern detector 12 in that device detects neither pattern signal, and the state of the switches of the device thus remain in the same state. The signal which was then supplied to the audio encoder 3 is encoded into an encoded audio signal, and passes through a second switch 20, proceeding to the cell assembly 4, so that the cell assembly 4 assembles a cell out of the signal supplied and outputs the cell to the line side.
When this cell arrives at a station which happens to be a called station for that call, the cell is disassembled by the cell disassembly 1 into an encoded audio signal. The resulting signal which was then inputted into the audio decoder 2 is decoded into a PCM audio signal therein, and subsequently given a first pattern signal in a first pattern inserter 10 before being outputted via a switch 21 to the exchange side. The signal is further transmitted to the telephone addressed by the call, which is connected to that exchange.
Also, when the cell arrives at a relay station, the arriving cell is processed similarly to the above in an audio tandem relay device of that station. This device, i.e., a relay device, is connected to another audio tandem relay device via an exchange, and these two devices make a pair in relaying, each hereinafter being referred to as a paired audio tandem relay device or a paired device. In the paired device, after it has received a signal from its exchange side, a first/second pattern detector 12 detects a first pattern signal in the signal received, upon which switches 20, 21 are switched so as to be connected at contact points 1. As a result, an output from the pseudo 64k signal decomposer 6a passes through the switch 20 and proceeds to the cell assembly 4, while an output of the pseudo 64k signal composer 5a passes through the switch 21 after being given a second pattern signal by the second pattern signal inserter 11, and is outputted to the exchange side. Note that a signal in the opposite direction in a telephone communication, which is a bidirectional communication, is similarly processed.
As described above, an audio signal is neither encoded nor decoded in a relay station, where composition or decomposition of a pseudo 64k signal is performed instead. In other words, an audio signal is encoded or decoded only in calling and called stations. With this arrangement, degradation of audio quality due to repetition of encoding/decoding can be effectively prevented.
FIG. 2 is a block diagram showing the structure of a conventional audio compression converter disclosed in JP laid-open No. Hei 7-307706, and so on. This structure comprises a silent data generator 30; a separator 31 for separating an audio signal (or an encoded audio signal) and a relay frame signal, both supplied from the exchange side; a multiplexer 32 for inserting a relay frame signal into an audio signal (or an encoded audio signal); an audio encoder 33 for encoding an audio signal; an audio decoder 34 for decoding an encoded audio signal inputted; selectors 35, 36; a relay frame detector 37 for detecting a relay frame signal which has been separated by the separator 31; a relay frame inserter 38 for inserting a relay frame signal; and a timer 39.
The operation will next be described based on the accompanying drawings. In normal call connection, the selectors 35, 36 select outputs from the audio encoder 33 and the audio decoder 34, respectively. With this, an audio signal supplied from the exchange side is encoded by the audio encoder 33 and outputted as an encoded audio signal to the line side, while an encoded audio signal from the line side is decoded by the audio encoder 34 and multiplexed with a relay frame signal by the multiplexer 32 before being outputted to the exchange side.
In relay call connection, on the other hand, the relay frame detector 37 detects a relay frame signal, upon which the selector 35 is switched so as to select silent data for output from the silent data generator 30. A predetermined period of time after the detection, the timer 39 switches the selectors 35, 36 such that they select and output signals which have bypassed the audio encoder 33 and the audio decoder 34, respectively. With this arrangement, an audio signal is not encoded or decoded in a relay station, and is only encoded or decoded in calling and called stations. This can prevent degradation of audio quality due to repetition of audio encoding/decoding.
However, a conventional audio tandem relay device shown in FIG. 1 has such a problem that callers may sometimes hear loud noises. To be specific, in relay connection where one audio tandem relay device of a pair detects either a first or second pattern signal later or earlier than the other, loud noises may be produced and heard by the caller. This happens because one device of a pair will stay in a relay connection state in which the first and second switches 21, 20 remain connected at the contact points 1 until both detect either a first or a second pattern signal, whereas the other is then in a calling state in which they are connected at the contact points 2. In short, the difference in detection timing for first and second pattern signals between the two devices will hinder uniform changing of the states of these devices. This results in the aforementioned problem of loud noises.
Also, in a conventional audio compression converter, if relay frame signals are detected at different timings by a pair of such converters in relay connection, decoded audio data is outputted to a caller without being encoded by its pair converter. This also causes a problem of loud noises.